Reorganize docs into category folders and drop the retired head-review page

Group the top-level docs into docs/project, docs/design, docs/assets and
docs/process, each with its own README index. Delete the superseded
docs/screens/head-review page and sheets. Rewrite every markdown link,
absolute docs path and tool/test reference to the new locations, fix the
pre-existing broken anchors and remove the dead Joke Shop photo-board link.
check_docs OK, Python tooling 167/167.

src/test/capture_npc_pose.gd

@@ -23,7 +23,7 @@ extends SceneTree
 ## same `_animate()` sequence *does* render the crouch, and writing a single
 ## bone rotation directly does deform the mesh, so this is a staging/capture
 ## discrepancy, not a rig failure. Do not use these sheets as pose evidence.
-## Repro and what was ruled out: `docs/3d-model-review.md` section
+## Repro and what was ruled out: `docs/assets/3d-model-review.md` section
 ## "Pose evidence is blocked".
 ##
 ## Usage (windowed; `xvfb-run -a` works for automation):